In the present invention, the position of a positron source, typically within the body for purposes of medical diagnosis, is determined by detection of the pair of oppositely traveling gamma rays produced by electron collision with the emitted positrons after only a short trajectory. By simultaneously detecting the location of the two gamma rays, positional information on the positron source is obtainable.
Typically, detection of the gamma rays is accomplished using a gamma radiation scintillating crystal such as sodium iodide crystal with the resulting photo emission detected by a photomultiplier tube. In some systems each photomultiplier has a single crystal associated with it, leaving gaps between the crystals. In order to improve the resolution of the system, the entire ensemble of crystals and photomultiplier tubes is moved in a time dependent fashion. Where the activity under investigation is a time varying phenomena, this time dependent detection will produce a blurring and result in a loss of diagnostic information.
An example of another system is shown in Tanaka, "Scintillation Cameras Based On New Position Arithmetic", Journal of Nuclear Medicine, Volume 11, No.9. A single large crystal spans several photomultipliers and uses a delay line to transpose position and time to locate a scintillation point. This system, however, is limited in the rate at which events can be processed.
Another problem from the use of a large single crystal is due to the uncertainty of the actual depth within the crystal at which scintillation occurs. The depth of scintillation affects the dispersion of the light ultimately received by the photodetectors. This in turn varies the output of the photodetectors independently of gamma ray trajectory. Additionally, when a scintillation occurs near an edge of the crystal, edge effects will alter the light dispersion pattern received by the photodetectors and thus tend to distort the sensing of the position of the scintillation point.